Cyclic nucleotide phosphodiesterases: important signaling modulators and therapeutic targets

Abstract

By catalyzing hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), cyclic nucleotide phosphodiesterases are critical regulators of their intracellular concentrations and their biological effects. As these intracellular second messengers control many cellular homeostatic processes, dysregulation of their signals and signaling pathways initiate or modulate pathophysiological pathways related to various disease states, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication, chronic obstructive pulmonary disease, and psoriasis. Alterations in expression of PDEs and PDE-gene mutations (especially mutations in PDE6, PDE8B, PDE11A, and PDE4) have been implicated in various diseases and cancer pathologies. PDEs also play important role in formation and function of multimolecular signaling/regulatory complexes, called signalosomes. At specific intracellular locations, individual PDEs, together with pathway-specific signaling molecules, regulators, and effectors, are incorporated into specific signalosomes, where they facilitate and regulate compartmentalization of cyclic nucleotide signaling pathways and specific cellular functions. Currently, only a limited number of PDE inhibitors (PDE3, PDE4, PDE5 inhibitors) are used in clinical practice. Future paths to novel drug discovery include the crystal structure-based design approach, which has resulted in generation of more effective family-selective inhibitors, as well as burgeoning development of strategies to alter compartmentalized cyclic nucleotide signaling pathways by selectively targeting individual PDEs and their signalosome partners.

Keywords: A-kinase anchoring protein (AKAP); cAMP; cancer; oral and systemic diseases; phosphodiesterase (PDE); protein kinase A (PKA); signalosome.

Figure 1. Cyclic-nucleotide mediated signal transduction of phosphodiesterases

The PDE superfamily contains 11 highly regulated and structurally related gene families. Some PDE's are specific for cAMP (PDE-4, -7, -8), some are cGMP specific (PDE-5, -6, -9), and some exhibit mixed specificity (PDE-1, -2, -3, -10, -11). Individual families contain closely-related subfamilies, e.g., PDE1A, PDE1B, PDE1C. Cyclic nucleotides are generated through activation of cyclases in response to a variety of regulatory signals or hydrolyzed by phosphodiesterases. Activation of cAMP- and cGMP-dependent protein kinases: protein kinase A (PKA) and protein kinase G (PKG) leads to the phosphorylation of specific substrate proteins that initiate and regulate cyclic nucleotide-dependent and kinase-mediated signaling pathways and their biological effects. Some effects of cyclic nucleotides are mediated by binding to and interaction with specific proteins such as cAMP-GEF, which activate RAP1-pathways. CNG ion channels activated by cAMP and cGMP are important cellular switches and transduce changes in intracellular cyclic nucleotides into changes in membrane potential. PDE's can be activated by binding of cAMP or cGMP to non catalytic sites.

Figure 2. Domain organization of different PDE gene families

The N-terminal part of the PDE gene family (shown in black) contains targeting domains responsible for localization of PDE isoforms to specific subcellular sites of and signalosomes. The N-terminal regulatory domain (shown in green) contains PDE family-specific subdomains [allosteric ligand binding sites, i.e. GAF domain for PDE2,5,6,10,11; Ca+2/Calmodulin-binding site for PDE1; transmembrane domain in PDE3; upstream conserved regions (UCRs) in PDE4; per-arnt-sim (PAS) and the Y-homologous receiver domain (REC) in PDE8]. N-terminal hydrophobic regions (NHRs) in PDE3 are important in regulating subcellular localization and formation of compartmentalized signalosomes. The conserved catalytic domain (shown in blue) is located in the carboxy-terminal part of the PDEs. PDE3 catalytic domain contains a unique 44-amino-acid insert (shown in red).

Figure 3. PDE8A regulates Raf-1 and Erk signaling network

Raf is a key activator of Erk signaling pathways. AKAP-LBC positions PKA for a preferred role in cAMP/PKA-induced phosphorylation of S259-Raf1, which inhibits its kinase activity, and consequently, inhibits activation of erk and erk-signaling. Raf-1 promotes assembly of the PDE8A/Raf-1 signalosome, leading to PKA-induced phosphorylation and activation of PDE8A, thus allowing PDE8A to reduce cAMP and thus limit PKA-mediated phosphorylation of Raf-1 at S259 and limit inhibition of Erk signaling.

Figure 4. AKAPs: Scaffolds for PDE-containing signalosomes

The subcellular localization of different AKAPs is shown. Compartmentalization of cyclic nucleotide signaling pathways involves the specific subcellular targeting/tethering of scaffolding molecules (i.e. AKAPs), effectors of cyclic nucleotides (phosphodiesterases, protein kinase A, protein kinase G, protein phosphatses, and substrates), and other signaling molecules.